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Ant choosiness reveals they all have different personalities

Well I beg to differ…

Norasmah Basari

By Chris Simms

The power to imagine a better world has helped transform human societies, and it seems it may be doing so on a smaller scale to ant societies, too.

Individual ants have now been shown to have differences in behaviour − something almost akin to having a personality − that affect colony decisions. And some ants are so different in their personal preferences that they may act as the imagination of the colony, driving it on to a better future.

Rock ants (Temnothorax albipennis), found in coastal areas of the UK, make their homes in crevices. These openings are plentiful, but they can be easily damaged by rock slides or larger animals. If a nest is wrecked, or if scouts find better digs, it often makes more sense to relocate than it does to stay put.

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But not just any crevice will do. When looking for a new home, scouting ants have a high-maintenance list of requirements, says Thomas O’Shea-Wheller at the Ant Lab of the University of Bristol, UK. They seek low light levels, an entrance gap of 1 to 1.5 millimetres, a ceiling height of roughly 2 millimetres and an internal area of about 20 square centimetres.

Picky ants

To test what individuals think of potential nests and how this might affect a group decision to relocate, O’Shea-Wheller and his colleagues showed 160 individual ants – 16 from each of 10 colonies – artificial nests that were excellent, good or poor.

As is usually the case, the better the nest, the more time the ants spent in it laying down pheromones for other ants to detect.

“By remaining in a nest, ants effectively contribute to a quorum. The longer they spend there, the more likely it is other ants will join them,” says O’Shea-Wheller.

So far, so simple, but the team found a lot of variability between the amount of time individuals spent in a nest of a certain quality.

“Some ants are picky, others are more liberal and will accept almost anything as long as it is better than the existing nest,” says O’Shea-Wheller. “Much like humans, not everyone wants to live in a mansion.”

Imagination of the colony

And some ants never seem happy, however nice a nest is, he says. They live there, but seem restless, and are more likely to scout. That might seem like a problem, but it means they are always searching for new things. “They are the imagination of the colony,” says O’Shea-Wheller.

“It would be a bit of a jump to say they had personality, but it shows that there is variance in the way they make a decision,” he says.

Others are more relaxed about what we call this variance. “I’m happy using ‘personality’ for any behavioural differences,” says Anna Dornhaus, who studies social insects at the University of Arizona in Tucson.

The team modelled this variation and found that in choosing between two nests of different standards, it seemed to make little difference. Whether the ants acted like mindless clones or variable viewers, the optimal decision would be reached at pretty much the same speed.

But if the colony was choosing between two poor nests, the heterogeneity of responses to the same nests meant the colony would come to a collective decision quicker. The ants with more extreme behaviour, in this case the ones that would settle for almost anything, helped make the collective decision-making process faster and more flexible.

If several of these ants remain in one of these poor nests, or recruit others to the nest, it pushes the colony to move.

“In any decision-making there is a trade-off between speed and accuracy,” says Nathalie Stroeymeyt at the University of Lausanne in Switzerland. “Heterogeneity is faster but less accurate. When the nests are all low quality, heterogeneity allows them to make a decision if many ants are picky. Heterogeneity is really the key to their flexibility – this is the first evidence of that.”

Learning from experience

O’Shea-Wheller’s team also varied the order in which each ant saw the three different quality nests, revealing that individual ants can be influenced by prior experience.

“We found that if they went from excellent to good they perceived the good one as not as good as if they had seen it first,” says O’Shea-Wheller.

A similar pattern was seen when the nest order was reversed. Ants that saw a poor nest first spent even longer in a good nest they saw next.

Such “learning” has been seen at the level of the colony before, for example in relation to avoiding poor nests that have been previously scouted. But this is the first time individual ants have been shown to acquire information and use it to modulate future behaviour, the team says.

“It is efficient for them to change their quality rating based on what they’ve seen. They need to set a point of what to expect that works in the changing environment,” says Dornhaus.

Individuality is important

“This adds to the evidence that individuality is important,” says Stroeymeyt. “It’s really cool because it does an individual-level investigation that explains something that happens at the colony level.”

Overall, this work shows that having individual ants make different decisions may be as important as how the tasks are divided throughout the colony, says O’Shea-Wheller.

“People have long known about division of labour in insects. We find it intuitive that specialising is a good thing. This shows that you can also find benefits at the group level when the behaviour is random, not coordinated,” says Dornhaus. “The ability of the colony to find new nest sites depends on there being some wanting to search. It’s useful to the colony to have some ants that are fussy.”

But what’s behind these differences? “It could be down to all sorts of factors,” says Dornhaus. “A difference in a hormone level, the number of sensory receptors they have, age, body size. It’s hard to know.”

“We’d like to know what drives personality differences, what the evolutionary benefits are,” she says. “At least this gives us a suggestion about why personality differences could be useful – and could benefit a colony.”